Abstract
The passivation treatment on type 304 stainless steel was applied at 313K for 3.6ks in 1.5-6.0kmol·m-3 HNO3 solutions which include 0-5×10-2kmol·m-3 NaF. After the treatment, pitting resistance of the treated steel was evaluated by measuring pitting potential at 313K in 1.0kmol·m-3 NaCl solution with a potential sweep rate of 0.83mV·s-1. The most remarkable improvement of pitting potential was found at the condition of 1.5 or 3.0kmol·m-3 HNO3-5×10-3 kmol·m-3 NaF solutions. The XPS analysis of the passive film revealed that the enrichment of Cr in the passive film and the increase of film thickness, which were responsible to the remarkable improvement in the pitting resistance. The experimental results in the present work suggest that the fluoride ions in the passive film accelerate outward migration of iron in the film and the substrate to the solution, resulting in the enrichment of Cr in the film and the increase of thickness of passive film.
